Description of the Prior Art
Fluidized catalytic cracking of heavy petroleum fractions is one of the major refining methods to convert crude or partially refined petroleum oil to useful products, such as fuels for internal combustion engines and heating oils. In such fluidized catalytic cracking, (known popularly as "FCC") high molecular weight hydrocarbon liquids are contacted with hot, finely divided solid catalyst particles in an elongated riser or transfer line reactor. The reactor is usually in the form of a riser tube and the contact time of the material is on the order of a few seconds, say one to ten seconds, and generally not over about three seconds. This short contact time is necessary to optimize generation of gasoline and middle distillate fractions. By proper selection of temperatures and reaction times the catalytic cracking reaction is "quenched" so that economically undesirable end products of such a reaction, methane and carbon, are held to a minimum, and yield of desired products, gasoline and middle distillate oils, is at a maximum. During this short reaction period a hydrocarbon feed stock, frequently in the form of vacuum gas oil, cycle oil or the like, at an initial temperature of from about 300.degree. F. to 800.degree. F., is sprayed onto catalyst at temperatures in the range of about 1100.degree. F. to 1400.degree. F. The present invention, as noted above is particularly directed to a system of uniformly misting such feed onto the hot catalyst.
Generally the mixture is fluidized partially by steam, but primarily by hydrocarbon gases that evolve by the hydrocarbonaceous feed vaporizing upon contact with the hot catalyst. Reaction of the mixture is one of essentially instantaneous generation of large volumes of gaseous hydrocarbons. The hydrocarbon vapors and catalyst mixture flow out of the riser tube into a separator or disengaging vessel. The spent catalyst is separated, primarily by gravity and inertia forces acting on the catalyst in the separator vessel, and passed downwardly through a stripper section for return to a regenerator. Fluidizing steam also generally flows up through the down-flowing catalyst to assist in stripping hydrocarbon vapor from the spent catalyst. Heat for the process is obtained by burning the coke, primarily carbon, on the spent catalyst by flowing oxygen through a bed of spent catalyst in a regenerator vessel. The regenerated and heated catalyst is then recirculated to the riser reactor. The desired product, hydrocarbon vapor, is recovered overhead from the separator vessel. Generally, this recovery is through one or more cyclone separators connected to a plenum chamber or common piping and directly piped to a distillation column. Vapor flow through the cyclone separators extracts residual or entrained catalyst fines. The catalyst fines are recovered from the cyclone separators through "dip legs" connected to the spent catalyst stripper, at the bottom or below the disengaging vessel, for return to the regenerator.
A particular problem in the initial generation of hydrocarbon vapor is that if the hydrocarbon liquid does not directly contact catalyst upon injection into the reactor riser, thermal cracking appears to be favored over the catalytic reaction. Such thermal cracking tends to generate end products of methane and coke. That is, complete conversion of hydrocarbons in the feed produces gas and coke, rather than desired middle distillate hydrocarbons. Prolonged contact of the unvaporized liquid hydrocarbons with catalyst after discharge into a separation vessel may result in further thermal cracking which tends to favor such end reactions particularly at high velocities. Further, it is essential to such catalytic cracking that hydrocarbon vapor contacts the catalyst because such reaction is primarily a vapor phase reaction.
While it has been proposed heretofore to use misting or fine droplet nozzles in the riser reactor pipe, in general such fine dispersions have been obtained by the use of steam or other vaporizing materials which form a two phase fluid. A particular problem with such two-phase fluids is that in general they produce a higher pressure drop through the spray nozzles than either fluid phase alone. This is important because pressure drop across the nozzle unit for a given size nozzle and a given rate of feed has a significant influence on the size of droplets that can be formed by the nozzle. It is of course also undesirable to add additional steam to the hydrocarbon feed. Such added steam must be recovered in the overhead distillation column and generally creates a "sour" water disposal problem, because oxides of sulfur, nitrogen, and carbon in the recovered hydrocarbon vapors combine with the water to form acids. In spite of such problems, steam has been used heretofore primarily because it reduces the hydrocarbon partial pressure and accordingly reduces resistance to vaporization of the feed stream by the catalyst.
U.S. Pat. No. 3,152,065--Sharp et al, U.S. Pat. No. 3,812,029--Snyder, U.S. Pat. No. 3,654,140--Griffel et al, and U.S. Pat. No. 3,071,540--McMahon et al are examples of feed nozzles for fluid catalytic cracking systems in which steam or water is concurrently injected with the hydrocarbon feed through an annular area surrounding the hydrocarbon feed nozzle. These patents indicate the advantages of using a "shroud" of steam around or within a nozzle disposed directly in the riser reactor for spraying hydrocarbon feed into the catalyst flow stream.
In Sharp et al, the feed is swirled by spiral vanes positioned around a straight flow pipe carrying steam into a liquid-steam mixing chamber. Release of the mixture from the chamber is through a sharp or square-edged orifice which is only slightly larger in diameter than the steam tube diameter. This is said to form a annular wall of liquid material, with misting of the hydrocarbon feed resulting from the steam forming a hollow conical spray of liquid that has it genesis at the free end, or orifice, of the nozzle. The nozzle is positioned directly in the catalyst flow stream. Additionally, the patentees disclose flow of steam to which centrifugal action is also imparted around the heated feed flow as well as the inner steam flow pipe.
The Snyder et al patent discloses hydrocarbon feed flowing through a surrounding water nozzle which concurrently cools the feed nozzle to prevent coking and disperses the mixture of water and feed into finer droplets.
Griffel et al disclose the use of a venturi in the supply line to disperse the feed. The nozzle is disposed within the riser reactor for combined steam and hydrocarbon feed flow. Alternatively, the patentees disclose a spiral member in the hydrocarbon feed nozzle itself to impart a centrifugal component to the feed which is released through a straight tube. A surrounding flow of steam induces breakup of the flowing hydrocarbon feed to droplets.
In the arrangement shown by McMahon et al, steam and hydrocarbon liquid are fed concentrically through a nozzle arrangement. This is similar to apparatus disclosed by Snyder for concurrent water and hydrocarbon liquid. The concentric nozzles are positioned in the center of the rise reactor with annular flow of catalyst particles around the nozzles.
U.S. Pat. No. 4,097,243--Bartholic--discloses a hydrocarbon feed distributor in which a divergent conical header supplies a center nozzle and a plurality of surrounding divergent nozzles. The feed distributor or header is disposed in the center of a riser reactor with catalyst flow around the nozzle.
U.S. Pat. No. 3,848,811--Fryback--discloses a fluid discharge nozzle for injecting hydrocarbon feed into a riser reactor as a plurality of discreet concentric streams. A plurality of circumferentially spaced holes diverge outwardly relative to the direction of flow through the nozzle, as do a pair of frusto-conical members arranged in line with the direction of flow. One of the frusto-conical members includes additional port members so that in general feed is sprayed from a multiplicity of nozzles all directed generally outwardly and upwardly from the nozzle into the riser pipe. The nozzle is positioned in the center of the riser reactor to contact catalyst flowing downwardly over the nozzle, with steam flowing upwardly with catalyst around the nozzle.
U.S. Pat. No. 2,786,801--McKinley et al--discloses an arrangement for spraying hydrocarbon feed into fluidized catalyst from a nozzle within a shroud positioned directly in the bed or stream. Catalyst flow is either upward or downward relative to the shroud. The nozzle arrangements include systems for (1) spraying liquid only, or liquid and gas either (2) separately or (3) combined. However, each nozzle includes a tapered throat leading either to a tube or conical discharge opening. The conical discharge opening either converges, or diverges as a venturi from the tapered throat. The pantentees indicate that the only requirement is that the nozzle be encompassed within a shroud. The patent also suggests that a shroud having the nozzle inside, may be located on the wall of the reactor tube.
U.S. Pat. No. 2,937,988--Polack--discloses a system for feeding oil and steam separately through concentric nozzles disposed in or directly above an annular flow diverter in the steam of solid particles. The steam nozzle may be omitted, but where used it is indicated to pass the steam transversely across the oil flow, thus shearing and atomizing the oil. The arrangement is directed to preventing direct contact of the feed with side walls of the reactor or transfer line.
U.S. Pat. No. 2,698,284--Adams--discloses adding water to a residuum feed stream to assist dispersion in a coking operation in a levitated bed of coke, sand or the like. A spray nozzle is positioned in the center of the reactor and within the bed.
U.S. Pat. No. 2,994,659--Slyngstad et al--discloses in FIG. 3 combining steam with hydrocarbon feed to a fluid catalytic cracking system through a nozzle within a reactor tube. However, the liquid-steam feed nozzle is surrounded by an annular nozzle carrying steam. Additionally, fluffing steam is injected around the annular steam nozzle.
None of the foregoing patents discloses a hydrocarbon feed system wherein a single fluid stream, with or without steam included therein, is misted into a flowing stream of fluidized catalyst particles by generating a free vortex in a single fluid hydrocarbon stream by passing it through a centrifugal acceleration chamber, including vanes, and then releasing the full flow through a sharp or square-edge discharge orifice recessed sufficiently within the reactor side wall so that the discharge orifice itself is out of the stream of fluidized catalyst, but so positioned that the vena contracta of fluid flowing from the orifice is sufficiently close to maintain its solid liquid flow form into the catalyst stream. Such location of the nozzle orifice in the side wall assures that the liquid stream breaks up into a fine mist over a conical pattern well within the catalyst stream. Such mounting assures that the outer surface of the nozzle is not coked by the feed or abraded (with eventual destruction of the metal nozzle) by high velocity catalyst particles made of exceedingly abrasive compounds, such as alumina and/or silica.